The present invention relates to ion vapor deposition of (IVD) aluminum or magnesium onto steel/aluminum alloy. More particularly, this invention relates to an improved sacrificial galvanic protection for steel in mild environments such as inland, rural and mild industrial atmospheres.
In the past, ion vapor deposited aluminum has been used to provide sacrificial galvanic protection to steel and aluminum in severe environments, such as marine or heavy industrial atmospheres. However, when steel structures having ion vapor deposited aluminum coatings are exposed to milder environments, such as inland, rural, and mild industrial atmospheres, the aluminum coating forms an oxide film which is unduly passive and suppresses galvanic activity. This is particularly true when steel is used in the landing gear and critical structural components of aircraft and/or rocket motor cases.
Sacrificial galvanic protection has also been employed on offshore steel structures. Several mechanisms have been proposed for such galvanic activity.
One such proposed activation mechanism for these aluminum anodes involves alloying zinc, indium, tin and/or mercury with aluminum. The zinc, indium, or tin will locally separate the aluminum oxide film. These alloying elements are customarily used in aluminum cathodic protection anodes for sea-going structures. They have not been used as corrosion protection coatings up to now. A proposed mechanism for the effect of these alloying elements is described in an article entitled "A Proposed Activation Mechanism For Aluminum Anodes" by M. C. Reboul, et. al. published in Corrosion Magazine, Volume 40, No. 7 (July 1984), pages 366-371.
Another mechanism consists of an impressed direct current between the offshore steel structures and particular anodes made from the aluminum and the above described alloys. The current polarizes the structures, shifting their potential.
However, attempts to adapt these mechanisms to ion vapor deposition have heretofore been untried. For example, it is technologically prohibitive to apply anodes generating galvanic current on the surface of aircraft and/or rocket components, and little is known as to whether it would be effective to vapor plate multiple metallic components over a steel substrate so as to provide galvanic activity.
Of all the metal coating processes, ion vapor deposition is commonly considered the process providing the most secure bonding mechanism between metal substrates and the metal coating. The bond is characterized by relatively deep difusion between the molecules of the two metals at the interface between those metals. The effectiveness of the bond to a large measure derives from the ability to thoroughly clean the substrate as an adjunct of the ion vapor deposition process and to deposit the metal coating in a powerful ion bombardment. A trough-like boat is employed as an evaporator unit to receive the coating metal, and a temperature control mechanism for heating the boat sufficiently to melt and evaporate the coating metal, is commonly employed both of which are located within a vacuum chamber of the ion vapor deposition devices.
Owing to the wide variation between the melting and vapor points of aluminum as compared to the melting and vapor points of other metals which are anodic to steel such as zinc and indium, or nonanodic metals such as tin, separate auxilliary evaporation boats with independent temperature controls were believed necessary to ion vapor deposit aluminum with the other alloying elements.
That is to say, aluminum melts at approximately 660 degrees C. and vaporizes at approximately 2,467 degrees C., while zinc melts at only 419 degrees C. and vaporizes at but 907 degrees C. All the same, indium melts at only 156 degrees C., while it takes a temperature of 2,080 degrees C. before it vaporizes. Tin, likewise, melts at a relatively low temperature of 232 degrees C., while not vaporizing until 2,270 degrees C.
However, we have discovered that the "shields" employed in IVD processes can prevent cross-contamination between the independent boats, actually lowers the coating efficiency, because the evaporants condense on the surface of the shields. Accordingly, it did not appear possible to produce a galvanically active alloyed IVD aluminum coating.
Accordingly, a method for electrochemical activation of IVD aluminum coatings that would remain stable against the formation of passive oxides in mild environments has here-to-fore, not been possible. An effective method, which would overcome the drawbacks previously encountered, would represent a substantial advancement in the art.